Interleukin-1.beta. converting enzyme, which is referred to herein as ICE.alpha., is a cytoplasmic cysteine protease that cleaves inactive 31 kDa proIL-1.beta. to generate the active 17.5 kDa proinflammatory cytokine IL-1.beta. (Black, R. A., et al. (1989) FEBS Lett. 247, 386-390; and Kostura, M. J., et al. (1989) Proc. Natl. Acad. Sci. USA 86, 5227-5231, which are each incorporated herein by reference). ICE.alpha. is expressed in many tissues as an inactive proenzyme polypeptide of 404 amino acids (SEQ ID NO:2) and a relative molecular mass (Mr) of 45 kDa (p45) (Cerretti, D. P., et al. (1992) Science 256, 97-100; and Thornberry, N. A., et al. (1992) Nature 356, 768-774) .
Active ICE.alpha. is produced after proteolytic cleavage of the proenzyme p45 to generate two subunits of Mr 20 kDa and 10 kDa, known as p20 and p10 subunits. Recent crystal structure analysis of active ICES demonstrated that the two subunits associate with each other to form a (p20).sub.2 /(P10).sub.2 tetramer (Wilson, K. P., et al. (1994) Nature 370, 270-275) also referred to as a (p20/P10).sub.2 homodimer (Walker, N. P. C., et al. (1994) Cell 78, 343-352).
The structure of ICE.alpha. is unique and is not related to any known protein structures including those of other cysteine proteases. ICE.alpha. is also unusual in its substrate specificity. ICE.alpha. requires an Asp in the P1 position and a small preferably hydrophobic residue in the P1' position (Sleath, P. R., et al. (1990) J. Biol. Chem. 265, 14526-14528; and Howard, A. D., et al. (1991) J. Immunol. 147, 2964-2969). Only the serine protease granzyme B and its homologs have a similar requirement for Asp in the P1 position.
Sequence homology between ICE.alpha. and the Caenorhabditis elegans cell death gene product CED-3 suggests that mammalian ICE.alpha. or its homologs might be involved in apoptosis. The two proteins share an overall 28% sequence identity (Yuan, J., et al. (1993) Cell 75, 641-652). A 43% identity is observed when a region which contains the enzyme active site is compared. A significant homology between ICE.alpha. or CED-3, and a newly discovered mouse protein known as Nedd2 was also demonstrated in a recent study and the significance of this homology to CED-3 was demonstrated when overexpression of ICES or Nedd2 in fibroblasts resulted in apoptosis (Kumar, S., et al. (1994) Genes & Develpoment 8, 1613-1626; and Miura, M., et al. (1993) Cell 75, 653-660). Expression of crmA, a poxvirus specific inhibitor of ICE.alpha. (Ray, C., et al. (1992) Cell 69, 597-604) was able to block ICE.alpha. apoptosis in fibroblasts and to protect ganglion neurons from apoptosis induced by nerve growth factor depletion (Gagliardini, V., et al. (1994) Science 263, 826-828).
There is a need for compounds which inhibit activity of ICE isoforms. There is a need for kits and methods of identifying such compounds. There is a need for isolated ICE isoforms, and for compositions and methods of producing and isolating ICE isoforms.